Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor substrates. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation may be used to manufacture solar cells. A solar cell is a device having an in-built electric field that separates the charge carriers generated through the absorption of photons in the semiconductor material. This electric field is typically created through the formation of a p-n junction (diode), which is created by differential doping of the semiconductor material. Doping a part of the solar cell substrate (e.g. surface region) with impurities of opposite polarity forms a p-n junction that may be used as a photovoltaic device converting light into electricity.
FIG. 1 shows a first embodiment of a solar cell, and is a cross section of a representative substrate 150. Photons 160 enter the solar cell 150 through the top surface 162, as signified by the arrows. These photons 160 pass through an anti-reflective coating 152, designed to maximize the number of photons 160 that penetrate the substrate 150 and minimize those that are reflected away from the substrate 150.
Internally, the solar cell 150 is formed so as to have a p-n junction 170. This p-n junction 170 is shown as being substantially parallel to the top surface 162 of the substrate 150 although there are other implementations where the p-n junction 170 may not be parallel to the top surface 162. The solar cell 150 is fabricated such that the photons 160 enter the solar cell 150 through a heavily doped region, also known as the emitter 153. In some embodiments, the emitter 153 may be an n-type doped region, while in other embodiments, the emitter 153 may be a p-type doped region. The photons 160 with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the semiconductor material's valence band to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band. In order to generate a photocurrent that can drive an external load, these electron hole (e-h) pairs need to be separated. This is done through the built-in electric field at the p-n junction 170. Thus any e-h pairs that are generated in the depletion region of the p-n junction 170 get separated, as are any other minority carriers that diffuse to the depletion region of the solar cell 150. Since a majority of the incident photons 160 are absorbed in near surface regions of the solar cell 150, the minority carriers generated in the emitter 153 need to diffuse across the depth of the emitter 153 to reach the depletion region and get swept across to the other side. Thus to maximize the collection of photo-generated current and minimize the chances of carrier recombination in the emitter 153, it is preferable to have the emitter 153 that is shallow.
Some photons 160 pass through the emitter 153 and enter the base 154. In the scenario where the emitter 153 is an n-type region, the base 154 is a p-type doped region. These photons 160 can then excite electrons within the base 154, which are free to move into the emitter 153, while the associated holes remain in the base 154. Alternatively, in the case where the emitter 153 is a p-type doped region, the base is an n-type doped region. In this case, these photons 160 can then excite electrons within the base 154, which remain in the base 154, while the associated holes move into the emitter 153. As a result of the charge separation caused by the presence of this p-n junction 170, the extra carriers (electrons and holes) generated by the photons can then be used to drive an external load to complete the circuit.
By externally connecting the emitter 153 to the base 154 through an external load, it is possible to conduct current and therefore provide power. To achieve this, contacts 151 and 155, typically metallic, are placed on the outer surface of the emitter 153 and the base 154, respectively. Since the base 154 does not receive the photons 160 directly, typically its contact 155 is placed along the entire outer surface. In contrast, the outer surface of the emitter 153 receives photons 160 and therefore cannot be completely covered with contacts 151. However, if the electrons have to travel great distances to the contact 151, the series resistance of the solar cell 150 increases, which lowers the power output. In an attempt to balance these two considerations (the distance that the free electrons must travel to the contact 151, and the amount of exposed emitter surface 163) most applications use contacts 151 that are in the form of fingers.
The embodiment shown in FIG. 1 requires contacts 151, 155 on both sides of the solar cell 150, thereby reducing the available area of the front surface through which photons 160 may pass. A cross section of a second embodiment of a solar cell 100 is shown in FIG. 2. Fundamentally, the physics of this embodiment is similar, in which a p-n junction is used to create an electric field which separates the generated electron hole pairs. However, rather than create the p-n junction across the entire surface, as done in the previous embodiment, the junctions are only created in portions of the solar cell 100. In this embodiment, a negatively doped silicon substrate (base 103) may be used. In certain embodiments, a more negatively biased front surface field (FSF) 102 is created by introducing addition n-type dopants in the front surface. This FSF 102 is then coated with an anti-reflective coating 101. This front, illuminated surface is often etched to create a pyramidal or other non-planar surface, so as to increase surface area. The metallic contacts, such as p contacts 107 and n contacts 108, and fingers 110 are all located on the bottom, non-illuminated surface of the solar cell 100. Certain portions of the bottom surface are doped with p-type dopants to create emitters 104. Other portions are doped with n-type dopants to create more negatively biased back surface field (BSF) 105. Typically, the regions between these emitters 104 and BSFs 105 are undoped and passivated with silicon oxide. The back surface is coated with a passivation layer 106 to enhance the reflectivity of the back surface. P contacts 107 and n contacts 108 are attached to the emitter 104 and the BSF 105. FIG. 3 shows one commonly used configuration where contacts are placed on the back non-illuminated surface. In this embodiment, the contacts are placed on one or more of the emitters 104 and the BSFs 105. This type of cell is known as a point contact solar cell.
With current energy costs and environmental concerns, solar cells are becoming more important globally. Any reduced cost to the manufacturing or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
The current manufacturing process for point contact solar cells requires at least two lithography and diffusion steps on the backside of the solar cell to fabricate the contact and emitter regions. Removing any process steps would reduce the manufacturing costs and complexity for the solar cells. Furthermore, any process steps that improve the efficiency of the solar cell would be beneficial. Accordingly, there is a need in the art for an improved method of doping point contact solar cells.